In providing for electronic instrumentation or industrial control, it is frequently necessary to apply a signal from a signal source, such as a sensor, thermocouple, or transducer, to electronic signal processing circuitry while maintaining complete electrical isolation between the signal source and the signal processing electronics. Various methods are presently used in constructing such isolation amplifiers, including transformer coupling. However, transformer coupling has required rather large and expensive transformers. Means for miniaturizing the transformer below certain limits have not been known.
A circuit in U.S. Pat. No. 4,191,929 to Max and Weedon illustrates the use of a transformer having center tapped windings in an isolation amplifier. A chopper is used to convert the analog input signal into an AC signal suitable for the primary of the isolation transformer. A synchronous chopper is connected to the secondary of the isolation transformer to restore the analog signal. The secondary winding of the isolation transformer is connected across the differential inputs of a high-gain amplifier. Negative feedback is provided around the amplifier so that the transformer secondary winding is connected across a virtual short; and the negative feedback tends to drive the voltage applied to the inputs of the amplifier to a very small value. The negative feedback from the amplifier induces a current in the transformer secondary which cancels the flux produced by current flowing in the primary. While this circuit does result in improved linearity, it does not fully solve the problems of transformer size and full isolation. The inability to permanently ground the transformer windings at a point which will minimize capacitive coupling between the windings prevents optimum operation of the circuitry. An additional difficulty with this circuit is that it places the demodulator between the secondary and the operational amplifier. This degrades the performance of the circuit by adding resistance and switching noise in the loop with the transformer secondary and the amplifier input impedance, thereby departing from the ideal situation of zero impedance, zero noise loading of the secondary.